Among known methods for identifying the failure point by simulation exploiting CAD, there are a back-tracing method and a failure simulating method by preparation of a failure dictionary.
In the failure simulation method by preparation of a failure dictionary, shown in FIG. 27, simulation is carried out on a normal LSI and for each of defined failure points. If any difference is produced in the results of simulation, the defined points at that time, output terminals where abnormalities occurred at that time, output values of the output terminals and test vectors are associated with one another and registered in the failure dictionary. In specifying the defect points of a defective item, data of the defective item (output terminals where abnormalities occurred, output values and the test vectors) and data for each failure point registered in the failure dictionary (output terminals where the abnormalities occurred, output values and the test vectors) are compared to one another. The failure point registered in association with data coincident with the data of the defective item is inferred to be a failure point.
In the back-tracing method, the failure point is identified by reverse logic tracing from an output terminal to an input terminal based on the output terminal where the abnormalities occurred, an output value and a test vector number. That is, if a pre-set signal is entered at an input terminal of the LSI, and an output signal at an output terminal differs from an expected output value, a signal propagating a failure is extracted from a signal internally propagated from the output terminal to the input terminal by exploiting the difference between the output value and the expected value for inferring the failure point. Then a failure is defined on the inferred failure point and a logical simulation is performed for verifying the coincidence with the actual failure for specifying the failure point. The usual practice is to check a plurality of abnormal output value points and limit pseudo failure signals based on the combination of the abnormal output value points to localize the failure point.